Zeolite L has been known for some time as an adsorbant, and in U.S. Pat. No. 3,216,789 is described as an aluminosilicate of the formula: EQU 0.9-1.3 M.sub.2/n O: Al.sub.2 O.sub.3 : 5.2-6.9 SiO.sub.2 : yH.sub.2 O
(where M is an exchangeable cation of valency n and y is from 0 to 9) having a characteristic X-ray diffraction pattern. The preparation of zeolite L is described in U.S. Pat. No. 3,216,789, EP-A-167755, EP-A-142355, EP-A-142347, EP-A-142349, EP-A-109199, PL-A-72149, U.S. Pat. No. 3,867,512, and SU-548567.
EP-A-96479 describes and claims zeolite L having a characteristic morphology and size, which is particularly valuable for use as a catalyst base in hydrocarbon conversions such as aromatisation, and comprising crystallites in the form of cylinders with a mean diameter of at least 0.1 micron, preferably at least 0.5 micron.
EP 96479 describes a synthesis of zeolite L which is conducted so that the amount of the contaminant zeolite W, which is know to grow in such a system as a competitive phase, is minimised. A preferred synthesis gel described in EP 96479 has the following mole ratios: EQU 2.62K.sub.2 O:Al.sub.2 O.sub.3 :10SiO.sub.2 :16OH.sub.2 O
and it is discussed how this gel may be varied by changing the molar amount of one component within the following ranges:
______________________________________ K.sub.2 O: 2.4-3.0 moles Al.sub.2 O.sub.3 : 0.6-1.3 moles SiO.sub.2 : 8-12 moles H.sub.2 O: 120-240 moles ______________________________________
EP-A-142353, EP-A-142354 and EP-A-185519 describe developments of this process for forming cylindrical zeolite L.
Zeolite L may be used as a catalyst base in aromatisation reactions. U.S. Pat. No. 4,104,320 discloses dehydrocyclisation of aliphatic compounds in the presence of hydrogen using a catalyst comprising zeolite L and a group VIII metal. The particular zeolite disclosed in EP 96479 is remarkably effective in such aromatisation reactions being capable of forming catalysts which have extended lifetime. Such dehydro- cyclisation and/or aromatisation reactions and catalysts for use in such reactions are also described in EP-A-107389, EP-A-184451, EP-A-142351, EP-A-145289, EP-A-184450, U.S. Pat. No. 4,614,834, GB-A-2116450, GB-A-2114150, U.S. Pat. No. 4,458,025, U.S. Pat. No. 4,456,527, GB-A-2142648, GB-A-2106483, U.S. Pat. No. 4,443,326, GB-A-2121427, GB-A-2153840, GB-A-2153384, U.S. Pat. No. 4,517,306, U.S. Pat. No. 4,539,304, U.S. Pat. No. 4,539,305, U.S. Pat. No. 4,547,472, GB-A-2166972, U.S. Pat. No. 4,579,831, U.S. Pat. No. 4,608,356 and EP-A-201856.